The present invention generally relates to copper-based alloys that are suitable for use in the production of castings (for example, plumbing castings), wrought forms (for example, produced by rolling, drawing, forging, etc.), and potentially other forms. The invention also relates to the production and processing of such alloys, and particularly processes that are capable of large-scale, efficient production of such alloys.
Copper-based alloys, and particularly brass and bronze alloys, are widely used in a variety of applications, notable but nonlimiting examples of which include plumbing systems. Relatively complex shapes (for example, valves) can be produced by casting brass and bronze alloys. Copper-based casting alloys that contain additions of metals having low melting points relative to copper tend to have very wide freezing ranges, which as used herein refers to the range of temperatures between the solidus temperature (below which the alloy is a solid) and liquidus temperature (above which the alloy is a liquid). The wide freezing ranges of such casting alloys give rise to extensive dendritic solidification and associated chemical segregation and microporosity. As particular examples, casting alloys most commonly used for plumbing applications contain tin (bronze alloys), zinc (brass alloys) and lead, all of which have relatively low melting temperatures compared to copper, with the result that these alloys have wide freezing ranges and are prone to extensive dendritic solidification, chemical segregation and microporosity.
Lead is insoluble in copper and has been used for many years in both cast and wrought copper alloys. Lead is widely considered to “plug” microporosity, which is largely due to the lead itself inducing a wide freezing range. More important, lead is known to improve machinability. In fact, leaded versions of brass and bronze alloys are specified for virtually all components requiring significant machining.
Plumbing component manufacturers are under increasing pressure to remove lead from valves and fittings, whether cast or wrought. As examples, the states of California and Vermont in the U.S.A. currently impose a limitation of 0.25% on the lead content in copper alloys for plumbing components in contact with potable water. In addition, there is a large environmental impact associated with the production of lead and lead-containing alloys. Refinery flue gasses disperse lead in the air and water, foundry furnaces generate airborne lead contaminants, requiring occupational monitoring (for example, blood testing), and lead contaminates the foundry molding sand, rendering it a hazardous solid waste.
The goal of developing lead-free and low-lead brasses and bronzes having the machinability of leaded alloys has been pursued for many years. Although there are many tests for measuring machinability and testing standards are often controversial, none of the alternative alloys developed to date are believed to meet or exceed the machinability of the leaded-brasses. Two main classes of alloys have emerged as the most viable substitutes for leaded brass. The first class encompasses what may be referred to as silicon-brass, which contains small additions of silicon. Commercial examples of these alloys were developed by Mitsubishi Shindoh Co. Ltd., and particular examples that are commercially available under the name Ecobrass® have been reported to have a nominal composition of, by weight, 75Cu-21Zn-3Si. The inclusion of silicon in this brass material results in the formation of hard second phase particles that facilitate chip breakage during machining. The second class encompasses alloys that contain bismuth (similar properties to lead) or both bismuth and selenium. Commercial examples include alloys available from Federal Alloys under the name Federalloy® and are formulated as Bi-substituted versions of common leaded casting brasses. The Federalloy® alloys produce structures akin to leaded brasses, in that bismuth phases form interdendritic pockets that facilitate chip breakage. Similar structures are produced in the Bi—Se alloys of this class. Commercial versions of these alloys, commonly known as “SeBiLoy,” often contain, by weight, about 0.5 to about 4% Bi and up to about 1% Se, and have been marketed under the name Envirobrass®. Although exposure to bismuth does not pose the same level of risk as lead, bismuth is a byproduct of lead production, is much more expensive, and is no longer produced in the US. Thus, bismuth-containing alloys do not appear to be an optimal long-term solution to the problem of replacing lead in brass.
Manganese is well known as an alloying element in commercial copper-based alloys, where it enjoys a reputation for enhancing properties for marine-based applications. Manganese bronzes, also known as high-strength yellow brasses (C86XXX) contain up to about 5 weight percent manganese, together with zinc, aluminum, nickel, iron and tin as alloying elements. Certain aluminum bronzes (C957XX) contain about 11 to about 14 weight percent manganese, together with aluminum, nickel and iron as main alloying elements. Two specialty alloys (C99700 and C99710) contain about 11 to about 23 weight percent manganese, together with high zinc concentrations, as well as nickel, iron and lead. Finally, another specialty alloy (C99600) known as Incramute 1, contains 39 to 45 weight percent manganese with 1 to 3 weight percent aluminum and smaller concentrations of other alloying elements.
Manganese-containing copper alloys have also been the subject of academic research. Two examples are Schievenbusch et al., “Directional Solidification of Near-azeotropic Cu Mn-alloys: a Model System for the Investigation of Morphology and Segregation Phenomena,” 1S1J International, Vol. 35, No. 6, p. 618-623 (1995), and Zimmermann et al., “Morphology and Segregation Behaviour in Directionally Solidified Copper-Manganese Alloys with Compositions Near the Melting Point Minimum,” Materials Science Forum Vol. 215-216, p. 133-140 (1996). These papers investigate Cu—Mn alloy compositions that undergo cellular and dendritic growth during directional solidification as a result of their compositions containing manganese contents that are intentionally above or below the “azeotrope” or (more properly) congruent point or minimum in the liquidus/solidus of the Cu—Mn phase diagram, shown in FIG. 1 (N. A. Goken, “Journal of Alloy Phase Equilibria,” 14 [1] p. 76-83 (1993)). Though there is uncertainty regarding the exact composition at the congruent point of the Cu—Mn system, Goken placed the congruent point at 34.6±1.4 weight percent (about 38±2 atomic percent) manganese. The particular focus of the investigations reported by Schievenbusch et al. was directional solidification experiments with alloy (Mn) concentrations within a range of ±5 weight percent around the azeotropic concentration, and the focus of the investigations reported by Zimmermann et al. used manganese concentrations of a few percent below and above the concentration of the melting point (azeotropic) minimum. The resulting microstructures were cellular as well as dendritic, evidenced by secondary arms developing in the microstructures.